The circulatory accessibility of airways receptors of known location will be studied injecting substances capable of changing their activity either into the systemic (left artium) or the pulmonary circulation (right atrium). Besides the verification of the widely accepted criterion of implying a more or less peripheral location of the intrapulmonary receptors on the basis of their preferential accessibility by either the pulmonary or the systemic circulation this study will indicate which blood (mixed venous or arterial) is more accessible to receptors of any particular location. This information has physiological relevance considering that several substances capable of affecting some of these endings have a different concentration in the blood of the pulmonary and systemic circulations. We will initiate a study of the capability of intrapulmonary slowly adapting receptors to originate signals related to the shape of the lung. For any given volume and/or transpulmonary pressure, the lung can have different configuarations and its slowly adapting stretch receptors could relate this information to the C.N.S.